1. Field of the Invention
The present invention relates to a cathode ray tube for displaying an image by forming a single picture plane by joining a plurality of split picture planes, and an intensity controlling method.
2. Description of the Related Art
At present, a cathode ray tube (CRT) is widely used in an image display apparatus (such as a television receiver, various monitors, and the like). In the CRT, an electron beam is emitted from an electron gun provided in the tube toward a phosphor screen and is electromagnetically deflected by a deflection yoke or the like, thereby forming a scan image according to the scan with the electron beam on the tube screen.
Generally, a CRT has a single electron gun. In recent years, a CRT having a plurality of electron guns is also being developed. For example, a gun type having two of electron guns for emitting three electron beams of red (R), green (G), and blue (B) has been developed (in-line electron gun type). In the CRT of the in-line electron gun type, a plurality of split picture planes are formed by a plurality of electron beams emitted from the plurality of electron guns and are joined, thereby displaying a single image. For example, the techniques related to the CRT of the in-line electron gun type are disclosed in Japanese Patent Laid-open No. Sho 50-17167, and the like. Such a CRT having a plurality of electron guns has an advantage that a larger screen can be achieved while reducing the depth as compared with a CRT using a single electron gun.
Methods of joining split picture planes in a CRT of the in-line electron gun type or the like includes a method of obtaining a single picture plane by linearly joining end portions of the split picture planes and a method of obtaining a single picture plane by partially overlapping neighboring split picture planes. FIGS. 1A and 1B show an example of obtaining a single picture plane by overlapping neighboring end portions of two split picture planes SR and SL as an example of forming a picture plane. In the example, the central portion of the picture plane is an overlapped area OL of the two split picture planes SR and SL.
In the CRT of the in-line electron gun type and the like, when a single picture plane is displayed by joining a plurality of split picture planes, it is desirable to make the joint of the split picture planes inconspicuous. Conventionally, however, the technique of making the joint inconspicuous has been insufficiently developed. For example, when the intensity at the joint portion is not properly adjusted, what is called intensity unevenness such that variation occurs in magnitude of intensity in the neighboring split picture planes. Conventionally, the technique of reducing the intensity unevenness has been insufficiently developed. In the case of obtaining a single picture plane by partially overlapping the neighboring split picture planes SR and SL as shown in FIGS. 1A and 1B, such intensity unevenness becomes a problem in the overlapped area OL of the neighboring split picture planes.
A method of reducing the intensity unevenness as described above is disclosed in, for example, the literature of SID digest, pp 351-354, 23.4: xe2x80x9cThe Camel CRTxe2x80x9d. The technique disclosed in the literature will be described by referring to FIGS. 1A and 1B. In the technique, a video signal corresponding to the overlapped area OL of the picture planes in a CRT is multiplied by a predetermined factor for correction in accordance with the position in the horizontal direction of a pixel (direction of overlapping the picture planes, that is, the X direction in FIG. 1B), that is, the signal level of an input signal is changed according to the direction of overlapping the picture planes and the resultant is output. In the method, for example, the level of the input signal for each of the picture planes corresponding to the overlapped area OL is corrected to have a sine function shape so that a value obtained by adding the intensity levels of input signals in the same pixel positions Pi.j (Pi.j1, Pi.j2) of the overlapped picture planes SL and SR is equal to the intensity in the same pixel position in an original image. However, such method has difficulty in improving the intensity in the entire intensity area, although the intensity can be improved in a part of an intensity area.
The problem in the conventional method of reducing the intensity unevenness will be described further in detail hereinbelow. Generally, the intensity Y of the screen in a CRT or the like is expressed by the following equation (1) when the level of an input signal is D and a characteristic value (gamma value) indicative of so-called gamma characteristic is xcex3. C is generally called perveance which is a coefficient determined according to the structure of the electronic gun or the like.
Y=Cxc3x97Dxcex3xe2x80x83xe2x80x83(1) 
The intensity distribution in the case where a single picture plane is formed by partially overlapping the two split picture planes like the example of FIGS. 1A and 1B will be considered. When gamma values in the two split picture planes SL and SR are xcex31 and xcex32, respectively, intensity Yxe2x80x21 and Yxe2x80x22 in the two split picture planes SL and SR in the overlapped area OL can be expressed by the following equations (2) and (3) similar to the above equation (1). In the equations (2) and (3), k1 and k2 are factors for correction by which the input signal D corresponding to the overlapped area OL in the picture plane is multiplied in accordance with the pixel position Pi.j. C1 and C2 denote predetermined coefficients corresponding to the coefficient C in the equation (1).
Yxe2x80x21=C1xc3x97(k1xc3x97D)xcex31xe2x80x83xe2x80x83(2) 
Yxe2x80x22=C2xc3x97(k2xc3x97D)xcex32xe2x80x83xe2x80x83(3) 
When the intensity in the two split picture planes SL and SR except for the overlapped area are Y1 and Y2, respectively, if the level of the input signal is the same in the entire area of the picture plane, the intensity is expected to be constant in the entire area of the picture plane. The condition under which the intensity unevenness does not occur can be expressed by the following equation (4). Yxe2x80x21+Yxe2x80x22 is a value obtained by adding the intensity values in the two split picture planes SL and SR in the overlapped area OL. When the equation (4) is solved, the following relational expression (5) is derived.
Y1=Y2=Yxe2x80x21+Yxe2x80x22xe2x80x83xe2x80x83(4) 
k1xcex31+k2xcex32=1xe2x80x83xe2x80x83(5) 
In the relational expression (5), when the gamma values xcex31 and xcex32 are fixed values, the factors k1 and k2 for correction can be unconditionally determined irrespective of the level of the input signal. In practice, however, as shown in FIG. 2, the gamma value depends on the level of the input signal and the intensity of the picture plane and is not constant.
The characteristic graph of FIG. 2 shows the relation between the level of an input signal (lateral axis) and the magnitude of intensity (cd/m2) actually measured on the screen (vertical axis). The graph is obtained by locally linearly connecting actual measurement points (indicated by painted dots xe2x80xa2 in the graph) each indicative of the value of the input signal and the value of intensity. In FIG. 2, the value of the input signal and the value of intensity are expressed as logarithm values. The gamma value xcex3 corresponds to the gradient of the graph (straight line). When the gradient of the graph is constant irrespective of the level of the input signal, the gamma value xcex3 is constant irrespective of the level of the input signal. In practice, however, the gradient of the graph varies according to the level of the input signal. It is therefore understood that the gamma value xcex3 varies according to the level of the input signal. Consequently, in order to satisfy the condition of the equation (5), a plurality of factors k1 and k2 for correction according to the level of an input signal are inherently necessary.
Particularly, in the case of a moving picture, usually, the level of the input signal dynamically changes. Consequently, it is desirable to control the intensity so that the factor for correction is dynamically to be an optimum one in accordance with the level of an input signal even in the same pixel position. In the conventional technique, however, the control of using a fixed factor irrespective of the level of the input signal is performed, and the control of dynamically changing the factor for correction in accordance with the level of the input signal is not performed. Conventionally, the intensity can be improved in a part of the intensity area, but not in the entire intensity area.
Japanese Patent Laid-open No. Hei 5-300452 discloses an invention to achieve smoothed intensity in the overlap area by preparing a plurality of smoothing curves for intensity control corresponding to the correction factors and selecting a curve according to the characteristic of an image projector or the like from the plurality of smoothing curves. According to the invention, the optimum curve is selected from the plurality of smoothing curves, information of the selected specific smoothing curve is stored in a non-volatile storage device, and the intensity is smoothed on the basis of the stored smoothing curve. In order to control the intensity in accordance with the signal level, a means for detecting the signal level is necessary. The publication however does not disclose or suggest the means for detecting the signal level. According to the invention disclosed in the publication, only the selected specific smoothing curve is stored in the non-volatile storage device. Therefore, the intensity cannot be dynamically adjusted while an image display apparatus is being used. In the invention disclosed in the publication, as long as a new smoothing curve is not stored in the nonvolatile storage device, the intensity control using the same smoothing curve is performed.
According to the invention of Japanese Patent Laid-open No. Hei 5-300452, therefore, the intensity control according to the signal level cannot be performed. The invention disclosed in the publication is a technique for optimizing the intensity adjustment performed mainly at the time of manufacture. The invention is not suited for performing the intensity control in a real-time manner while the device is being used. Although an analog control using the smoothing curve is carried out on a video signal in the invention disclosed in the publication, to adjust the intensity accurately, it is desirable to perform a digital intensity control using a correction factor independent for each unit pixel or unit pixel line. The invention disclosed in the publication is optimized for a projection type image display apparatus and is not suitable for display means for directly displaying an image by a scan with an electron beam like a cathode ray tube.
Since the gamma value xcex3 is influenced not only by the input signal but also by other factors, it is desirable to determine the factor for correcting intensity in consideration of the other various factors. For example, the gamma value xcex3 varies also according to colors. Consequently, in the case of displaying a color image, correction factors for respective colors are necessary. In a CRT, the characteristics of the gamma value xcex3 also vary according to characteristics of electron guns. It is therefore desirable to determine the correction factor in consideration of the characteristics of the electron gun and the like.
Further, as will be described hereinbelow, it is desirable to change the factor for correcting intensity in accordance with the position in the horizontal direction of a pixel (direction of overlapping the picture planes) and, in addition, in the perpendicular direction (the direction orthogonal to the direction of overlapping the picture planes, that is, the Y direction of FIG. 1B). The reason will be described by referring to FIGS. 1A and 1B. The intensity of a pixel in a position A (1A, 2A) and that of a pixel in a position B (1B, 2B) which are different from each other in the vertical direction in the overlapped area OL will be examined. When gamma values in positions 1A and 1B in the left-side split picture plane SL are set as xcex31A and xcex31B, respectively, intensity values Yxe2x80x21A and Yxe2x80x21B in the positions 1A and 1B obtained by performing a signal process using correction factors k1A and k1B on the input signal are expressed by the following equations (6) and (7), respectively, in a manner similar to the equation (1). C1A and C1B denote predetermined coefficients corresponding to the coefficient C in the equation (1).
Yxe2x80x21A=C1Axc3x97(k1Axc3x97D)xcex31Axe2x80x83xe2x80x83(6) 
Yxe2x80x21B=C1Bxc3x97(k1Bxc3x97D)xcex31Bxe2x80x83xe2x80x83(7) 
On the other hand, when gamma values in positions 2A and 2B in the right-side split picture plane SR are set as xcex32A and xcex32B, respectively, intensity values Yxe2x80x22A and Yxe2x80x22B in the positions 2A and 2B obtained by performing a signal process using correction factors k2A and k2B on the input signal D are expressed by the following equations (8) and (9), respectively. C2A and C2B denote predetermined coefficients corresponding to the coefficient C in the equation (1).
Yxe2x80x22A=C2Axc3x97(k2Axc3x97D)xcex32Axe2x80x83xe2x80x83(8) 
Yxe2x80x22B=C2Bxc3x97(k2Bxc3x97D)xcex32Bxe2x80x83xe2x80x83(9) 
When the intensity values in the positions 1A, 2A, 1B and 2B in the case of displaying an image only by a single electron gun are set as Y1A, Y2A, Y1B, and Y2B, respectively, the conditions under which no intensity unevenness occurs can be expressed by the following equations (10) and (11). Yxe2x80x21A+Yxe2x80x22A and Yxe2x80x21B+Yxe2x80x22B are values obtained by adding the intensity values of the two split picture planes SL and SR in the pixel positions A and B, respectively. When the equations (10) and (11) are solved, the following relational expressions (12) and (13) are derived, respectively.
Y1A=Y2A=Yxe2x80x21A+Yxe2x80x22Axe2x80x83xe2x80x83(10) 
Y1B=Y2B=Yxe2x80x21B+Yxe2x80x22Bxe2x80x83xe2x80x83(11) 
k1Axcex31A+k2Axcex32A=1xe2x80x83xe2x80x83(12) 
k1Bxcex31B+k2Bxcex32B=1xe2x80x83xe2x80x83(13) 
In a CRT, generally, transmittance of light and light generating efficiency vary according to the position of a pixel in a phosphor screen. The spot size of an electron beam or the like also varies according to the position of a pixel in the phosphor screen. Since the gamma value xcex3 varies according to the position of a pixel in the phosphor screen, the following equation (14) is therefore satisfied. Further, by the equations (12) to (14), the equation (15) is satisfied. It is understood from the equation (15) that it is preferable to control not only the intensity according to the position of a pixel in the horizontal direction as in the conventional technique but also the intensity in accordance with the position of a pixel in the vertical direction.
xcex31Axe2x89xa0xcex32A, xcex31Bxe2x89xa0xcex32Bxe2x80x83xe2x80x83(14) 
k1Axe2x89xa0k2A, k1Bxe2x89xa0k2Bxe2x80x83xe2x80x83(15) 
As described above, in order to perform an intensity control so as to make the joint portion inconspicuous from the viewpoint of intensity, desirably, factors for intensity correction are prepared for the pixel positions in the horizontal and vertical directions in the joint portion and at different signal levels, and the correction factor to be used for controlling the intensity is changed properly. To realize such intensity control, for example, there may be a method of pre-storing a number of correction factors according to the pixel positions, at different signal levels, and the like in the form of a table, and obtaining an optimum correction factor from the table in accordance with a change in the signal level or the like. However, when correction factors are prepared for all the pixel positions and at the all signal levels, the data amount becomes enormous. Such a method requires a work of pre-setting an optimum correction factor for each pixel position or signal level, so that it takes enormous time for the setting work occurs.
The present invention has been achieved in consideration of the problems and its object is to provide a cathode ray tube and an intensity controlling method that realizes the reduced number of factors for correcting intensity to be prepared in advance and can properly control the intensity so that the joint portion becomes inconspicuous from the viewpoint of intensity.
A cathode ray tube according to the invention includes: signal dividing means for dividing an input video signal into a plurality of video signals; first factor storing means for storing at least some of a plurality of first correction factors associated with signal levels of the video signals and pixel positions in a direction orthogonal to the overlapping direction, the some first correction factors being associated with representative pixel positions; and second factor storing means for storing at least some of a plurality of second correction factors associated with signal levels of the video signals and pixel positions in a overlapping direction, the some second correction factors being associated with the representative signal levels. The cathode ray tube according to the invention also has: first factor obtaining means for directly or indirectly obtaining a necessary first correction factor by using the first correction factors stored in the first factor storing means on the basis of a signal level of a present video signal and a pixel position in the orthogonal direction corresponding to the present video signal; changing means for changing a value of the signal level of a video signal referred to when the second correction factor is obtained on the basis of the first correction factor obtained by the first factor obtaining means; and second factor obtaining means for directly or indirectly obtaining the second correction factor to be used for intensity modulation control by using the second correction factor stored in the second factor storing means on the basis of the signal level changed by the changing means and the pixel position in the overlapping direction corresponding to the present video signal. The cathode ray tube according to the invention further includes: control means for performing the intensity modulation control on each of the video signals for the plurality of split picture planes so that a total of intensity values in the same pixel position in an overlapped area on the picture plane scanned based on the video signals for the plurality of split picture planes becomes equal to the intensity in the same pixel position in an original image by using the second correction factor obtained by the second factor obtaining means; and a plurality of electron guns for emitting a plurality of electron beams with which the plurality of split picture planes are scanned on the basis of a video signal modulated by the control means.
An intensity controlling method according to the present invention includes: a step of directly or indirectly obtaining a necessary first correction factor on the basis of the signal level of a present video signal and a pixel position in the orthogonal direction corresponding to the present video signal by using the first correction factors stored in the first factor storing means; a step of changing a value of the signal level of a video signal which is referred to when the second correction factor is obtained on the basis of the first correction factor obtained; a step of directly or indirectly obtaining a second correction factor to be used for intensity modulation control on the basis of the changed signal level and the pixel position in the overlapping direction corresponding to the present video signal by using the second correction factors stored in the second factor storing means; and a step of performing the intensity modulation control on each of the video signals for the plurality of split picture planes so that a total of intensity values in the same pixel position in an overlapped area on the picture plane scanned on the basis of the video signals for the plurality of split picture planes becomes equal to the intensity in the same pixel position in an original image by using the second correction factor obtained.
In the cathode ray tube and the intensity controlling method according to the invention, the first correction factor required is obtained directly or indirectly by using the first correction factors stored in the first factor storing means. And the value of the signal level of the video signal which is referred to when the second correction factor is obtained is changed on the basis of the first correction factor obtained. On the basis of the changed signal level and the pixel position in the overlapping direction corresponding to the present video signal, the second correction factor to be used for intensity modulation control is directly or indirectly obtained by using the second correction factors stored in the second factor storing means. By using the second correction factor obtained, the intensity modulation control is performed on each of the video signals for the plurality of split picture planes so that a total of intensity values in the same pixel position in an overlapped area on the picture plane scanned on the basis of the video signals for the plurality of split picture planes becomes equal to the intensity in the same pixel position in an original image.
Other and further objects, features and advantages of the invention will appear more fully from the following description.